The complete rat preproinsulin gene I (rI1) was cloned into an SV40 vector. Most of the late region of the viral vector (including the SV40 introns and all of the major splice junctions) was deleted and replaced by the entire rat insulin I gene. Analysis of the transcripts indicated that the rat preproinsulin gene nucleotide signals involved in RNA splicing and poly(A) addition were used. Examination of the 5' ends of the mRNAs showed several classes. This suggests that only a portion of the transcripts may be initiated faithfully. Significant quantities of a protein identified as rat proinsulin were synthesized, most of which is secreted. In an attempt to find a vector generating mRNA exclusively carrying authentic 5' end(s) (i.e. employing the cloned gene's promoter), a novel eukaryotic vector derived from the transforming region of bovine papillomavirus (BPV) was established and demonstrated to be highly effective for introducing foreign genes into animal cells. The foreign DNA is replicated as an episome, actively transcribed, and the transcripts are translated into an authentic gene product.